Magnetic nanoparticles with large surface to bulk ratio is a growing area of interest. Considering the potentially large area of application of magnetic nanoparticles, as filler materials of various polymer materials, it can easily be understood that their relatively poor representation in comparison to micron-sized filler materials in polymers is an effect of the difficulties related to the processing of high-surface area nanoparticles. The explanation mainly lies in the fact that large surface areas also brings problems in achieving uniformly distributed nanoparticle systems due to the favoured particle-particle interaction in comparison to particle-polymer/liquid interactions. The result is often severe agglomeration and aggregates of nanoparticles. The agglomerates in turn affect many macroscopic properties, such as mechanical, optical and magnetic etc. since these properties on a macroscopic scale are affected by the degree of close interaction at the nano scale level. In order to exploit the effects of nano-sized magnetic nanoparticles employed as fillers in organic matrix materials, the control over dispersion is therefore an unavoidable prerequisite.
Ferrite-loaded membranes of microfibrillated cellulose have been prepared by mixing metal ions to a suspension of bacterial cellulose under N2 atmosphere before precipitation by NaOH followed by oxidation in atmospheric air. Ferrite particles were inclined to aggregate into lumps in the fibrillar network Sourty H.; et al., Chem. Mater. 1998, 10 7), 1755-1757). A magnetic paper made of kenaf has been prepared by precipitation of magnetic nanoparticles in a pulp suspension under anaerobic conditions. Chia C. H. et al., Am. Appl. Sci., 2006, 3 3), 1750-1754).
Magnetic membranes with improved and controlled properties are of interest for purification/filtration (Dai Q., et al., Chem Soc Rev, 2010, 39, 4057), magneto-responsive actuators (Hoare, T. et al., Nano Lett, 2009, 9, 3651. Behrens S., Nanoscale, 2011, 3, 877) as well as for large scale manufacturing of e.g. magneto-acoustic membranes, anticounterfeiting papers, radio-frequency materials and flexible data storage. The magnetic nanocomposite membranes and films are classically derived from polymers mixed with surface modified functional magnetic nanoparticles (Behrens S., Nanoscale, 2011, 3, 877).
However, the dispersion of the high surface area nanoparticles is more challenging and nanoparticle agglomerates tend to form easily. Strength and failure properties are sensitive to such agglomerates so that the materials become brittle even at moderate nanoparticle loadings. The presence of agglomerates also makes it difficult to predict magnetic composite properties as related to intrinsic nanoparticle magnetics due to dipolar interactions (Olsson R. T., et al., Polym Eng Sci, 2011, Article in Press). In addition, the classical preparation methods (Behrens S., Nanoscale, 2011, 3, 877) are time consuming and costly since in most cases they rely on empirical attempts to find particle surface coatings for improved dispersions (Balazs A. C., et al., Science, 2006, 314, 1107).
Recent progress in the field of bio-nanotechnologies has shed light on the possibilities offered by some naturally occurring nano-building blocks (Eichorn S. J., et al. J Mater Sci, 2010, 45, 1). At the smallest scales of the wood cell wall organization, cellulose I microfibrils (3-5 nm wide) aggregate during wood pulping to form nanofibrils with dimensions in the range 5-20 nm in width and up to few micrometers in length. These entities can be released from the pulp fiber cell wall by mechanical disintegration (A. F. Turbak, et al., J Appl Polym Sci, 1983, 37, 815), which is facilitated by an enzymatic or chemical pre-treatment of the pulp fibers (M. Henriksson, et al., Eur Polym J, 2007, 43, 3434 and Saito T. et al., Biomacromolecules, 2007, 8, 2485). Due to their intrinsically high strength and stiffness (modulus of crystal exceeding 130 GPa (Sakurada I. et al., J Polym Sci, 1962, 57, 651)), long and slender cellulose nanofibrils (NFC) have interesting potential as nanoreinforcements in various composite materials. Furthermore, strong interfibril interactions allows formation of a variety of nanostructures, from dense nanopapersto ultra-light aerogels and foams (Henriksson, M. et al., Eur Polym J, 2007, 43, 3434; Pääkkö, M. et al., Soft Matter, 2008, 4, 2492; Sehaqui, H. et al., Soft Matter, 2010, 6, 1824; and Svagan, A. J. et al., J Mater Chem, 2010, 20, 6646). Here, the fibrillar interactions and the corresponding network structure provide favourable mechanical properties. Large-scale availability, origin from renewable resources, and low resource cost are advantages of forest-derived nano-building blocks.
Bacterial cellulose nanofibril networks have been used as a template for precipitation of magnetic nanoparticles (R. T. Olsson, et al., Nat Nanotechnol, 2010, 5, 584). The method allowed to form cellulose-based magnetic aerogels, as well as dense membranes. A two-step method for preparing a magnetic nanoparticle cellulose material, wherein cobalt ferrite nanoparticles are evenly/finely distributed arranged on the scaffold of fibres inside the material is disclosed in WO2008/121069. The disclosed material is in the form of a hydrogel or aerogel and the fibres in the material are physically entangled. However, the methods are energy consuming due to the freeze-drying steps of the cellulose network prior to nanoparticle precipitation. Furthermore, the versatility for nanostructure formation is restricted by the characteristics of the network synthesized by the bacteria, which to some extent predicted the relative density/frequency of the magnetic nanoparticles as related to reactive sites for grafting the inorganic nanoparticles.
A common problem with previous methods is to achieve reproducibly coated nanoparticles, making the combined mechanical and magnetic functionality of “classical” polymer matrix nanocomposites difficult to achieve. There is a need within the technical field of magnetic nanoparticle cellulose material to be able to tailor the magnetic properties of the material.